The present invention relates to a sealed battery, in which the leaking of electrolyte can be checked, and in particular, to a lithium ion battery, in which leaking can be checked at high sensitivity and in reliable manner.
In a sealed battery such as lithium ion battery, a battery element is placed in a battery case, and a header with an external connection terminal, a safety valve, an electrolyte inlet, etc. is attached on the battery case by welding. Then, the battery case is sealed except the electrolyte inlet, and after the electrolyte is injected, a sealing piece is attached on the electrolyte inlet, and the inlet is sealed by laser welding.
During the manufacture of the sealed battery, detects may occur such as welding defect caused in the sealing process of the inlet or defective caulking on the external connection terminal mounted via an insulating member. As a result, the electrolyte may leak or moisture in external air may be introduced into the battery. In the sealed battery with defective sealing, problems may arise such as damage of devices due to leakage of the electrolyte as the performance characteristics of the battery are deteriorated.
In this respect, leakage is checked with the purpose of preventing the shipment of the battery with defective sealing. It has been generally practiced to perform external inspection by visually checking the battery charged after sealing and by confirming stains or the like on the surface caused by the leakage of the electrolyte.
However, in the checking method based on visual inspection, there may be variations due to individual difference of the inspectors. When there is very small sealing defect, it is very difficult to detect a battery with such small sealing defect because the time for inspection is limited at the time of shipment and because the leaking amount is very small.
In this connection, a method for airtightness test using an airtightness tester not depending on human vision has been proposed. For example, a method is proposed in JP-A-9-115555. According to this method, the battery to be tested is accommodated in a sealed container, and gas pressure inside is reduced and pressure change in the sealed container is measured. By this method based on the pressure change, it is not possible to detect the battery with leakage accurately within short time.
According to JP-A-4-25738, an airtightness tester is described. In this tester, after a top cover is mounted on a battery chamber in the assembling process of the battery, hydrogen is introduced under pressure into the battery chamber, and leaking hydrogen is detected by a semiconductor gas sensor. In this method, it is not that leakage is tested in a completed sealed battery with the electrolyte injected in it. Also, an explosive gas such as hydrogen is used, and this leads to a problem of endangering safety.
JP-A-11-307136 describes an airtightness tester. In this tester, after a top cover is attached on a battery chamber, a cover is mounted to enclose junction of the battery chamber and the top cover. Then, helium is introduced into the battery chamber. The air in the cover is sucked and airtightness is judged by detecting helium in the sucked air. This is based on the test of the junction between the battery chamber and the top cover in the manufacturing process, and it is not a leakage test of a completed sealed battery with the electrolyte injected in it.
It is an object of the present invention to provide a sealed battery, by which it is possible to reliably detect defective sealing point of a sealed battery. It is another object of the invention to provide a method for manufacturing a sealed battery, by which it is possible to completely prevent the shipment of the battery with defective sealing.